The circuit will regulate the rms output voltage across the load (a projection lamp) to 100 volts Â±2% for an input voltage between 105 and 250 volts ac. This is accomplished by indirectly sensing the light output of lamp LI and applying this feedback signal to the firing circuit (Ql and Q2) which controls the conduction angle of TRIAC Q3. The lamp voltage is provided by TRIAC Q3, whose conduction angle is set by the firing circuit for unijunction transistor Q2. The circuit is synchronized with the line through the full-wave bridge rectifier.
The voltage to the firing circuit is limited by zener diode D5. Phase control of the supply voltage is set by the charging rate of capacitor Cl. Q2 will fire when the voltage on Cl reaches approximately 0.65 times the zener voltage. The charging rate of Cl is set by the conduction of Ql, which is controlled by the resistance of photocell R2. Potentiometers R3 and R4 are used to set the lamp voltage to 100 volts when the line voltage is 105 volts and 250 volts, respectively.